Valve for injection molding

ABSTRACT

A valve for controlling the flow of a high pressure fluid is provided. The valve provides a reliable way of controlling the flow of high pressure fluids in applications requiring multiple cycles where equipment may be exposed to environmentally harsh conditions, such as high temperatures and pressures.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/710,756, filed Nov. 10, 2000, by Kim et al., titled “Valve forInjection Molding,” which claims priority to U.S. ProvisionalApplication Serial No. 60/242,866, filed Oct. 24, 2000, by Kim et al.,titled “Valve for Injection Molding,” the disclosures of whichapplication are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to a valve for controlling the flow ofa high pressure fluid and, in particular, a valve for use in high cyclepolymer operations.

BACKGROUND OF THE INVENTION

The flow control of high pressure fluids is important in a variety ofindustries, including manufacturing, chemical production, refrigeration,plastics molding, automotive and medical applications. In the plasticsmolding industry, for example, fluids are often transferred attemperatures of several hundred degrees F and at pressures of severalthousand psi.

In several methods of polymer production, described in InternationalPublication No. WO 98/31521 (Pierick et. al.), International PublicationNo. WO 99/32544 (Anderson et. al.), and International Publication No. WO98/08667 (Burnham, et. al.), each of which is hereby incorporated byreference herein, a gas blowing agent is mixed with a molten polymer toproduce a mixture of gas and polymer. These processes may be used toproduce, for example, injection molded, blow molded or extrudedpolymeric materials. Typically, a high pressure gas is injected andmixed with a molten polymer prior to the polymer being molded orextruded. Certain processes, e.g. injection molding, involve producingproduct cyclically, e.g., with many parts being made sequentially in thesame mold. In such cases it may be preferred that the high pressure gascan be isolated from the polymer stream during the time the polymer isbeing transferred to a mold.

High pressure fluid valves used in cyclic operations are typicallysubjected to strenuous mechanically harsh conditions. It is one goal ofthe invention to provide a robust high-pressure valve for such use.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a valve comprising a valve seat, avalve pin having a drive end and a sealing end, the valve pin mounted inthe valve such that the sealing end is capable of mating with the valveseat, and a packing washer supporting the valve pin, the packing washerhaving a first end and a second end. An internal diameter of the packingwasher varies between the first end and the second end.

In another aspect, the invention provides a valve comprising a valveseat positioned in a valve body, a valve pin positioned in the valvebody, the valve pin having a sealing end and a drive end. The sealingend is capable of mating with the valve seat to prevent flow of fluidthrough the valve. A piston stem is positioned in a housing, the pistonstem having a first end and a second end and capable of transferringaxial force to the valve pin. The piston stem is uncoupled from thevalve pin.

In another aspect, the invention provides a valve comprising a valveseat positioned in a valve body, a valve pin positioned in the valvebody. The valve pin has a sealing end and a drive end. The sealing endis configured to mate with the valve seat to prevent flow of fluidthrough the valve. An energized seal surrounds a portion of the valvepin, and prevents flow of fluid past the seal.

In another aspect, the invention provides a valve comprising a valveseat, a valve pin capable of forming a fluid-tight seal with the valveseat, and a valve pin guide adjacent the valve seat. The valve pin guidehas an internal diameter that is substantially the same as the outerdiameter of the valve pin.

In another aspect, the invention provides a valve comprising a valveseat mounted in a valve body, and a valve pin having a sealing end and adrive end. The valve pin is slidably mounted in the valve body andaxially movable between a first position wherein the sealing end ismated with the valve seat and a second position wherein fluid can flowthrough the valve. A valve guide is positioned adjacent to the valveseat, and an energized seal surrounds a portion of the valve pin. Apiston stem moves the valve pin from the second position to the firstposition, and the piston stem is capable of transmitting axial force tothe valve pin while permitting the axis of the piston stem to moveindependently of the axis of the valve pin.

In another aspect, the invention provides a system comprising any of theabove valves in communication with a polymeric foam processing apparatussuch as extrusion, injection molding, or blow molding apparatus.

Other advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings, which areschematic and which are not intended to be drawn to scale. In thefigures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in cross section a prior art gas injection valve.

FIG. 2 illustrates in cross section one embodiment of a fluid injectionvalve according to the invention.

FIG. 3 illustrates in cross section an enlarged view of a seal of thefluid injection valve illustrated in FIG. 2.

FIG. 4 illustrates in cross section, an enlarged view of the seal areaof an embodiment of the invention.

FIG. 5 illustrates in cross section, an enlarged view of the seal areaof another embodiment of the invention.

FIG. 6 illustrates in cross section, a view of an embodiment of apolymeric processing system according to the invention.

DETAILED DESCRIPTION

An example of a typical prior art valve used to control the flow of ahigh pressure fluid, such as compressed nitrogen, into a polymer mix, isillustrated in FIG. 1. This description is not meant to imply thatother, similar arrangements are not present in the prior art. The valveincludes a valve body 10 having a valve seat 20 supported in the valvebody. Valve pin 30 is slidably mounted in the valve body 10 so that thesealing end 32 of the valve pin can mate with valve seat 20 and preventthe flow of gas through the valve body and out of gas exit 40 that maylead to a polymeric foam processing system. The diameter of the valvepin is uniform until the conical taper at the sealing end, and theinternal diameter of the valve body is uniform throughout. Typically,the difference between these two diameters is large enough, at least0.03 inch, to allow an adequate flow of compressed gas between the valvepin 30 and the interior of the valve body 10 when the valve is opened.The valve seat may be composed of 316 SS and the valve pin of 17-4hardened steel. Although, in all embodiments described herein, the valveof the invention is described as being able to prevent the flow of fluidin one position, while allowing fluid to flow in another position, thevalve can easily be modified by those of ordinary skill in the art withthe benefit of the present disclosure to inhibit (although notcompletely prevent) the flow of fluid in the first position whileallowing fluid to flow in the second position. The valve can easily beconstructed to allow for control of fluid flow between and amongessentially any fluid flow levels from complete prevention of fluid flowto free fluid flow in the valve's completely “open” position.Accordingly, in all embodiments, the valve can allow a first level offluid flow in a first position and a second level of fluid flow(different from the first level of flow) in a second position. Either ofthe first or second levels of fluid flow can define complete preventionof fluid flow.

Valve pin 30 is supported in the valve body by upper washer 50, lowerwasher 60 and fluoropolymer seal 70. Upper washer 50 is adjustable vianut 55 and serves to compress seal 70 between upper washer 50 and lowerwasher 60. The internal diameter of upper washer 50 is consistent and isdesigned to close tolerances to precisely guide valve pin 30 as thevalve pin slides to open and close the valve. For example, the internaldiameter of upper washer 50 can be less than 0.0005 inch greater thanthe diameter of valve pin 30. Thus, upper washer 50 may be used to alignand direct valve pin 30.

Valve pin 30 is swaged to piston stem 80 at junction 81 so that anymovement to piston stem 80 is translated to valve pin 30. Thus, ifpiston stem 80 moves downward (referring to FIG. 1), valve pin 30 alsomoves downward, to a position where sealing end 32 mates with valve seat20 to form a pressure tight seal. If piston stem 80 moves upward, valvepin 30 is pulled away from valve seat 20 and gas may flow through inlet12, through valve body cavity 14 between the wall of the cavity and thevalve pin, through a gate between pin sealing end 32 and valve seat 20,and through exit 40 that is in communication with a polymeric foamprocessing system. Thus, by controlling the movement of piston 110 bysupplying compressed air to either chamber 112 or chamber 114, themovement of piston stem 80, and therefore valve pin 30, is controlled.When the valve is to be closed, compressed air is directed to chamber112. When the valve is to be opened, compressed air is fed to chamber114.

When high pressure gas is injected through inlet 12 into the valve body,it is important for the system to be sealed to prevent the escape ofhigh pressure gas. This can be facilitated to some extent via PTFE seal70 placed around valve pin 30 to form a seal between the inner wall ofvalve body 10 and valve pin 30. As the PTFE seal may start to “coldflow” or “creep,” upper washer 50 can be tightened and therebycompressed against adjustable seal 70, to expand the seal in order tominimize leakage past the seal. When the seal has flowed to such anextent that it is no longer capable of sealing, it can be replaced. PTFEis a material that can provide a seal and allow the reciprocation ofvalve pin 30 between the open and closed positions. Reciprocation maytypically involve a travel distance of 0.125 inch or greater.

The valve illustrated in FIG. 1 may work adequately in a high pressurepolymeric foam processing system, however, it also may fail after about20,000 cycles, defining a limited lifetime. Not only does seal 70require repeated tightening and replacement, but an elliptical hole canbecome worn into valve seat 20, thus allowing gas to escape past theseat through exit 40 when the valve is in the closed position. When usedwith a polymeric foam processing system, this is disadvantageous as highpressure gas is allowed to pass through the processing system whendownstream gates are opened. Frequent replacement of the valve seat maybe required with the system of FIG. 1.

The inventors have found several reasons that may contribute to thepremature failure of the prior art valve of FIG. 1. For example, as theadjustable seal 70 flows and is repeatedly tightened, the thickness ofthe seal begins to decrease. At this point, the upper washer 50 maycontact the interior surface of valve body 10 and become misaligned, asshelf 16 is typically not exactly perpendicular to the axis of valve pin30. This means that the bore of upper washer 50 may no longer beproperly aligned with the axial movement of valve pin 30, and becausevalve pin 30 is carefully fit into upper washer 50 at very lowtolerances, any change in this angle forces the valve pin away fromcenter so that conical valve tip 32 is no longer axially aligned withvalve seat 20. As conical valve tip 20 is cycled repeatedly off center,it begins to unevenly wear an edge of valve seat 20 where the valve pinfirst comes into contact with the valve seat upon closure. In addition,because valve pin 30 is swaged to piston stem 80, any change in theangle of direction of movement of piston stem 80 urges a similar changein the motion of valve stem 30, resulting in additional forces that canresult in misalignment between valve pin 30 and valve seat 20. Thus, theoverall rigidity of the valve components, previously thought to providea repeatable, long-lasting system capable of maintaining alignment underhigh pressure and high temperature conditions, actually contributes tothe valve pin misalignment and the resulting failure of the valveitself.

The present invention provides a valve that has been shown to provideconsistent sealing of high pressure fluid through more than 20,000cycles of an injection molding system without the need to adjust sealsor change valve seats. Preferred valves of the invention can provideconsistent sealing through 100,000, 500,000, 1 million, 2 million, orgreater than 2.5 million cycles. One embodiment of the invention isprovided in FIG. 2. FIG. 2 provides an illustration of an embodiment ofa fluid control valve 200 that can control the flow of fluids, includinggases and supercritical fluids, and provide consistent repeatablecycling at temperatures of at least about 650° F. and pressures greaterthan 6,000 psi. Valve 200 includes a valve pin 230, a valve seat 220shaped to receive a sealing end 232 of the pin, a piston stem 280, apiston 310 operatively linked to the stem and moveable within a cylinder311, and a ball 320 positioned to transfer a force from stem 280 to pin230. Valve pin 230, which can be cylindrical, can be composed of atemperature robust material, e.g., a metal or alloy such as tungstencarbide, and has a sealing end 232 designed to mate with valve seat 220and a drive end 234 opposite to sealing end 232. Sealing end 232 may beany shape that can form a fluid type seal when in contact with seat 220and may be conical, as shown in FIG. 2, or, for example, can behemispherical. In operation, sealing end 232 slides into a valve guide245, which may be a section of chamber 214 that is narrower than is thesection above it.

Valve guide 245 may be designed to close tolerances, with very littleclearance between valve pin 230 and the walls of valve guide 245. Theguide may be an integral part of valve body 210 or, alternatively, maybe a separate insert. Preferably, the inner diameter of valve guide 245is substantially the same as the diameter of valve pin 230. Valve guide245 may extend up to the full length of interior surface 216, but it ispreferred that the valve guide be as short as possible while stillconstraining valve pin 230 when valve pin 230 is fully withdrawn fromvalve seat 220. In this way, resistance to fluid flow between inlet 212and outlet 240 is minimized, as is friction between valve pin 230 andvalve guide 245. In this manner, sealing end 232 is maintained in properalignment with valve seat 220 due to the close tolerances between valveguide 245 and valve pin 230.

In order to ease the passage of high pressure fluid through inlet 212 tochamber 214 and out of exit 240 when the valve pin is in the upper, oropen position, especially when valve guide 245 is present, fluidchannels may be provided within or proximate the valve guide. One ormore channels may be formed in the interior wall of valve guide 245, forexample, by using EDM techniques known to those skilled in the art, or,alternatively, one or more shapes, such as flats or concavities, may bemachined into the surface of valve pin 230 in the area of end 232. Forexample, flat 238 may be machined into valve pin 230 to provide a fluidpathway between valve pin 230 and valve guide 245. Preferably, two ormore flats are symmetrically opposed to each other.

Opposite of end 232 of valve pin 230 is drive end 234 which may includea broadened contact area as illustrated in FIG. 2, the surface of whichmay be either flat, as illustrated in FIG. 2 or, alternatively, concaveor convex. Shown just above drive end 234 is ball 320 which may be madeout of any suitable material, including tungsten carbide or hardenedsteel. Ball 320 serves as a point source to transfer force from pistonstem 280 to the drive end of valve pin 230. Piston stem 280 may bedriven by any device capable of providing force, including a pneumaticpiston, as illustrated in FIG. 2, a hydraulic piston, or an electricallypowered actuator. Ball 320 may be contained between piston stem 280 andvalve pin 230 without being affixed to either of the components withwhich it makes contact. In this manner, when piston stem 280 iswithdrawn, ball 320 may move freely upward until retained by a stop,such as shoulder 292. In alternative embodiments, the ball, or anotherpoint source, may form an integral part of either piston stem 280 orvalve pin 230, without being affixed to the other. It is preferable,however, that force be transferred from piston stem 280 to valve pin 230via a point source and that piston stem 280 and valve pin 230 not befixed together. In this manner, if piston stem 280 should become out ofaxial alignment with valve pin 230, or if a point source on either oneof the components should move off center, the driving force may still becleanly transferred from piston stem 280 to valve pin 230 withoutapplying undue torque to valve pin 230 that might cause premature weartoward one side of valve seat 220. The implementation of a stop, such asshoulder 292, provides for a preferred amount of minimal travel forvalve pin 230 while not requiring precise, limited movement of pistonstem 280. The stop may be positioned to limit the travel of the valvepin directly or can be positioned to limit the travel of the ball 320which, in turn, limits the travel of valve pin 230.

Valve pin 230, in conjunction with lower washer 260, energized seal 270,and packing washer 250, forms a fluid-tight seal to prevent flow offluid into cavity 291, which may be fluidly connected to the environmentoutside of the valve. Bottom washer 260 may be of any size and shapeappropriate for retaining seal 270 inside chamber 214. A gland 290 isused to hold packing washer 250 in place.

Energized seal 270 can be a nonadjustable seal that can form a fluidtight seal without external forces being applied to the top or bottom ofthe seal. Energized seal 270 can be, for instance, a “V” seal providinga sealing surface for both the interior and exterior of the seal. Theenergized seal should allow for the reciprocating movement of valve pin230 without allowing the passage of significant quantities of highpressure fluid. Energized seal 270, shown in cross-section in FIG. 3,may contain a spring 420 that provides an expanding force that serves toboth push the outer edge of energized seal 270 into contact withinterior surface 216 and also push the inner edge of energized seal 270into contact with the wall of valve pin 230. Spring 420 may be anyresilient substance that, in compression, provides an outward force.Preferably, spring 420 is a toroidal spring that forms a circle withinthe cylindrical seal. The contacting portion 410 of the energized sealthat is in contact with either the surface of the valve pin 230 or theinner wall of valve body 210 can be made out of any material capable offorming a fluid-tight seal under the high temperature and high pressureconditions encountered in the process with which the seal is used.Preferably, energized seal 270 is comprised of heat resistant elastomer,and more preferably is polymer/PTFE. In one embodiment, a seal fromParker GNP, referred to as type HS-11-008-S-106, has been shown toprovide acceptable results after 2.7 million cycles.

Packing washer 250 may be a flanged washer, as shown in FIG. 2, and mayserve to retain energized seal 270 as well as to center and align valvepin 230. Preferably, the inner bore of packing washer 250, through whichvalve pin 230 passes, is of different diameter at the upper end than atthe lower end. For example, the lower end of packing washer 250 may havean inner diameter very close to the diameter of the valve pin, and theupper end of packing washer 250 may have an inner diameter severalthousandths of an inch wider. This configuration still allows thepacking washer to help center valve pin 230. The variation in diameter,however, provides enough freedom of movement that any misalignmentbetween packing washer 250 and valve pin 230 will not result in a forcethat pushes valve pin 230 out of alignment with valve seat 220. Thisdesign helps to resolve the problem encountered with the valve of FIG. 1where any misalignment of the upper washer 50 tended to force the valvepin offline and to wear an elliptical hole in valve seat 20. In a mostpreferred embodiment, the inner diameter of packing washer 250 istapered at about 1.2° (exaggerated in FIGS. 2, 4 and 5), and morepreferably, the inner diameter is smaller at the bottom of the washerand larger at the top, although a reverse taper could functionsimilarly.

Valve 200 can also include a compression spring 236 providing an upwardforce to remove valve pin 230 from valve seat 220 when piston stem 280is retracted, especially when stem 280 and pin 230 are not affixed toeach other, i.e. not operatively linked to cause retraction of pin 230when stem 280 is retracted. The compression spring may be any type ofspring capable of providing a force to open the valve and may be madeof, for example, metal, polymer or resilient elastomer that is robustenough to withstand the environmental rigors of the application withwhich it is being used. Preferably, the reciprocal movement of valve pin230 is limited to less than about 0.1 inch and more preferably is in therange of 0.01 to 0.1 inch. In a most preferred embodiment, valve pin 230moves approximately 0.050 inch between the open and closed positions,the length of travel being controlled by shoulder 292.

In another embodiment, illustrated in FIG. 4, additional seals may beutilized to further isolate the high pressure fluid entering throughinlet 212 from cavity 291 and from the environment external to thevalve. In addition to lower washer 260 and energized seal 270,additional seals either upstream or downstream from the initial seal maybe added to make a more robust valve. The seals may be placed back toback or, alternatively, as shown in FIG. 4, a spacer such asintermediate washer 360 may be positioned between initial seal 270 andsecondary seal 370. Secondary seal 370 may be identical to initial seal270 or may be of a different design and different material. Appropriatematerials may include polytetrafluoroethylene (PTFE) and syntheticrubber, such as that sold under the trademark VITON® (DuPont DowElastomers, L.L.C.).

FIG. 5 illustrates another embodiment in which a secondary seal is usedto further isolate the high pressure fluid from the environment. Inaddition to lower washer 260 and initial seal 270, a secondary seal,O-ring 340, may be placed between the primary seal 270 and upper washer350. The O-ring may be placed directly behind initial seal 270, or anintermediate washer or other spacer may be placed between the two seals.The O-ring may be composed of any material capable of withstanding thepressure, temperature and wear conditions encountered by the seal. Forexample, 0 ring 340 may be composed of PTFE or synthetic rubber, such asthat sold under the trademark VITON® (DuPont Dow Elastomers, L.L.C.). Ofcourse, tertiary and additional seals may also be incorporated if foundto be useful in a specific machine, or process.

Referring back to FIG. 2, a chamber 312 defined within cylinder 311above the pistons may be connected to a source of high pressure air, forexample, air at 60 psi, in order to provide adequate force to pistonstem 280 in order to close valve 200. Compression spring 316 may provideadditional force to aid in the closure of the valve as high pressure gasentering inlet 212 can, in some arrangements, tend to push valve pin 230upward. In order to open the valve, a similar amount of air pressure maybe transmitted to chamber 312, and the force supplied in an upwarddirection from the compressed air, in combination with the forceprovided by the high pressure fluid entering the system at inlet 212, isgreat enough to overcome the force provided by spring 316, and thereforeis able to quickly open the valve.

The valve may be operated at high pressures and temperatures, andmaterials of construction should be chosen appropriately. A temperaturedifference of 300, 500 or more than 600° F. between various parts of thevalve may be encountered in some applications, including polymerprocessing applications. For instance, the valve may reach temperaturessurpassing 600° F. near an injector body but be considerably cooler, forexample, 140° F. near the air actuator. Parts subject to the conditionsnear the injector body should be made of suitable heat resistantmaterial. This high temperature operation may preclude the use ofpolymeric valve seats and components, and metals or ceramics may bepreferable. In addition, if the valve is used to control the flow ofsupercritical fluids (SCF), the fluid may act as a solvent and distortthe polymer over time. Polymers may also be prone to particulatecontamination, swelling, or distortion, due to heat. Metals are muchless prone to these problems. In addition, it is preferred that thevalve pin, seat and valve guide are of metal because the seat inpreferred embodiments should be able to endure millions of impact cyclesat high temperature, and the valve pin preferably is able to reciprocatecontinuously without galling. The choice of durable, heat resistantmaterials for the guide, valve pin and seat allow these components to bein close proximity to the polymer melt stream. Thus a valve comprising aseat, a guide and a valve pin of temperature resistant metal may bepreferred in a high temperature application. The drive components, suchas the piston and piston stem, may be placed farther from the heatsource and thus may not be subjected to the same extreme conditions.

Valve seat 220 and valve pin 230 may be made of the same or differentmaterials, however, to prevent galling, it is preferred that valve pin230 be of a harder material than is valve seat 220. It is preferred thatvalve seat 220 is made of a hard material, however, so that metalparticles are not embedded into the seat during operation. It ispreferred that the valve seat and valve pin be made of resilient,heat-resistant materials such as hardened steel, alloys, ceramics or,possibly, in some applications, high-temperature polymers. For example,valve seat 220 may be made of H1150 17-4PH hardened steel having aRockwell C hardness of about 33-36, while valve pin 230 may be made oftungsten carbide having a Rockwell C hardness of about 66 or greater.

In one aspect, the valve of the present invention may be used with apolymeric foam processing apparatus such as a blow molding, injectionmolding or extrusion molding machine. In one embodiment, illustrated inFIG. 6, valve 200 is used in conjunction with injection molding system500. The injection molding system includes a barrel 532 and a screw 538contained in the barrel for moving and mixing polymer melt. Alsoprovided is a drive motor 540 for driving the screw and a heating units542 for maintaining the barrel at an elevated temperature. Polymericmaterial is provided in hopper 544 and is fed into the extruder 514through orifice 546. A source 590 of blowing agent, which can be asupercritical fluid (SCF), is supplied to the extruder and the fluidflow is regulated by valve 200 which serves to selectively transportblowing agent from source 590 to blowing agent port 516, whichcommunicates with polymer processing space 534. SCF may be received intoblowing agent receiving section 562 and is mixed with polymer in mixingsection 560. The mixture of polymer and blowing agent then passesthrough a nucleating pathway 567 and through outlet 570 into mold 580.Valve 200 may be synchronized with the action of gate 564 and screw 538so that the supply of blowing agent is isolated from the extruder 514when gate 564 is open. Valve 200 may be opened when it is desirable toadd blowing agent to the polymer mix. Preferably, valve 200 is proximatepolymer processing space 534, so that when the valve is closed,substantially all of the high pressure fluid in communication with thepolymer has already entered the mixing section 560 of the extruder.

In such a configuration, valve 200 may be opened (see FIG. 2) to allowfluid, such as SCF, to pass through inlet 212 into cavity 214 andthrough outlet 240 in order to allow the fluid to mix with polymer. Whenthe polymer is to be injected into a mold, a force, for example, highpressure air, is supplied to cavity 312, supplying a downward force onpiston 310 and piston stem 280. The downward force is transferred toball 320 which then provides a point source to drive end 234 of valvepin 230. Valve pin 230 is in turn driven downwardly until sealing end232 is contacted with seat 220 in order to provide a fluid-tight seal.Once the fluid-tight seal is made, and the supply of high pressure fluidis cut off from exit 240, and polymer in the plastics production systemmay be injected into a mold cavity without the addition of, orinterference from, the fluid.

When the molding cycle is complete and additional gas is desired to bemixed with new polymer material, a signal is sent to an actuator, andhigh pressure air, for example at 60 psi, is provided to chamber 314within cylinder 311, below the piston, which is isolated by the pistonfrom chamber 312. The high pressure air in chamber 314 provides anupward force to piston 310 which in turn retracts attached piston stem280 upwardly. With piston stem 280 retracted, spring 236 can provide anadequate force, optionally in combination with that provided by any highpressure fluid in communication with the valve, to move valve pin 230and ball 320 upwardly until ball 320 is retained by shoulder 292 ingland 290. The amount of travel of pin 230 is controlled to be less thanthat which would be required for the pin to be fully extracted fromvalve pin guide 245. In this manner, the valve pin 230 remains in propercenter alignment for seating with valve seat 220 during the nextinjection cycle. Once an adequate supply of high pressure fluid haspassed through exit 240, the air source may be transferred from chamber314 back to chamber 312 to reinitiate the closing process.

One feature of preferred embodiments of the invention is that a valvecan include a piston that can move reciprocally to drive a valve pininto and out of a valve seat, where the piston is decoupled from thevalve pin. That is, the piston is not operatively linked to the valvepin in a way that components connecting the piston with the valve pinaffect alignment of the valve pin. Specifically, the piston drives apiston stem which, in turn, drives the valve pin (optionally via anauxiliary object such as a ball between the stem and pin), while theaxes of the stem and pin are free to move relative to each other. Onebenefit of this is that the piston and stem do not affect alignment ofthe valve pin relative to the valve seat. This feature, in combinationwith a preferred, internally-tapered packing washer, allows maximalfreedom of axial movement of the valve pin. The valve pin then can beguided, solely, by the valve pin guide, which will not come out ofalignment relative to the valve seat, and the valve pin thus is assuredof remaining in alignment with the valve seat.

Those skilled in the art would readily appreciate that all parameterslisted herein are meant to be exemplary and that actual parameters woulddepend upon the specific systems with which the invention is used. Itis, therefore, to be understood that the foregoing embodiments arepresented by way of example only and that, within the scope of theappended claims and equivalents thereto, the invention may be practicedotherwise than as specifically described.

What is claimed:
 1. A valve comprising: a valve seat; a valve pincapable of forming a fluid-tight seal with the valve seat; a valve pinguide adjacent the valve seat, the valve pin guide having an internaldiameter that is substantially the same as the outer diameter of thevalve pin; a packing washer supporting the valve pin, the packing washerhaving a first end and a second end, wherein an internal diameter of thepacking washer varies between the first end and the second end; and apiston stem having first and second ends and configured to transferaxial force to the valve pin, the piston stem being uncoupled from thevalve pin.
 2. The valve of claim 1 wherein the packing washer isinternally tapered.
 3. The valve of claim 1 further comprising a ballloosely positioned between the second end of the piston stem and a driveend of the valve pin, the ball capable of transferring force from thepiston stem to the valve pin.
 4. The valve of claim 1 comprising acompression spring in contact with the valve pin, the compression springproviding a force directing the valve pin away from the valve seat. 5.The valve of claim 1 comprising a stop to limit the travel of the valvepin in a direction toward the piston stem.
 6. The valve of claim 5wherein the stop is positioned to limit the travel of the valve pin toless than about 0.10 inch.
 7. The valve of claim 5 wherein the stop ispositioned to limit the travel of the valve pin to less than about 0.05inch.
 8. The valve of claim 1 further comprising an energized sealsurrounding a portion of the valve pin, the energized seal preventingflow of fluid past the seal.
 9. The valve of claim 8 wherein theenergized seal comprises a toroidal spring.
 10. The valve of claim 8wherein the energized seal is slideable in relation to the valve pin.11. The valve of claim 8 further comprising a second energized seal. 12.The valve of claim 8 comprising a secondary seal wherein the secondaryseal comprises an O-ring.
 13. The valve of claim 1 comprising acompression spring in contact with the valve pin, the compression springproviding a force directing the valve pin away from the valve seat. 14.A polymer processing system comprising: an extruder including a barreland a screw rotatable within the barrel, the barrel having a blowingagent port former therein; and a valve positioned between a blowingagent source and the blowing agent port, the valve comprising a valveseat, a valve pin capable of forming a fluid-tight seal with the valveseat, a valve pin guide adjacent the valve seat, the valve pin guidehaving an internal diameter that is substantially the same as the outerdiameter of the valve pin, a packing washer supporting the valve pin,the packing washer having a first end and a second end, wherein aninternal diameter of the packing washer varies between the first end andthe second end, and a piston stem having first and second ends andconfigured to transfer axial force to the valve pin, the piston stembeing uncoupled from the valve pin.
 15. The system of claim 14 furthercomprising a mold connected to an outlet of the extruder.